1. Field of the Invention
The invention relates to a pavement marker that reflects and/or transmits light for identifying selected regions of a driving surface.
2. Description of the Related Art
Pavement markers are secured on or in a pavement surface to help direct drivers along preferred travel lanes. For example, pavement markers often are mounted at selected intervals along the lane lines. The pavement marker includes a light reflective or light transmissive surface aligned to be readily apparent to a driver approaching the pavement marker. For example, the light reflective or light transmissive material in the pavement marker may be constructed to reflect or transmit ambient light toward the driver or to reflect headlights back toward the driver.
Pavement markers used in colder climates typically must be embedded at least partly into the pavement surface in view of the likely contact with a snow plow. Pavement markers used in warmer climates or on pavement surfaces where snow plow activity is not anticipated may be mounted on the surface of the pavement. Surface mounted pavement markers are substantially less expensive than pavement markers that must be embedded in the roadway surface and are much easier and less expensive to install. Hence, surface mounted pavement markers are preferred in situations where permitted by climate and roadway conditions.
Surface-mounted pavement markers frequently are struck by vehicular tires while the vehicle is traveling along the lane line or crossing the lane line at an angle that typically is less than 3°. The impact of tires moving over a pavement marker creates bending stresses and stress concentrations. Hence, the pavement marker must be sufficiently strong to maintain structural integrity in response to the frequent impact by vehicular tires.
Surface-mounted pavement markers also should be constructed to remain in a substantially fixed position on the surface of the pavement. In this regard, surface-mounted pavement markers typically are secured to the surface of the pavement by an adhesive. The adhesive must secure itself to both the pavement and to the pavement marker with sufficient force to resist movement in response to the impact imposed by vehicular tires. The adhesive used to affix a surface-mounted pavement marker typically is bitumen, which is a petroleum based product similar to the petroleum based products in the pavement. Bitumen never permanently hardens, and is subject to deformation, yielding and sheer, particularly in the warm weather environments in which surface-mounted pavement markers are most prevalent.
The strength of the attachment provided by the adhesive is affected largely by the surface area of contact between the adhesive and the pavement marker and to a lesser extent by the mechanical grip achieved by the adhesive. The strength of attachment attributable to the surface area of contact often is referred to as a chemical bond or a wetting parameter. The extent of the surface-to-surface contact between the adhesive and the pavement marker can be increased by providing a non-smooth bottom surface for the pavement marker. For example, pavement markers have been formed with an array of grooves in the bottom surface. Typically the grooves all are parallel to one another and are oriented to be substantially perpendicular to the lane lines. The adhesive applied to the roadway surface fills in the parallel grooves and increases the surface area of attachment between the adhesive and the pavement marker. Additionally, grooves aligned perpendicular to the lane lines provide some minor mechanical gripping that resists forces generated in response to impact by tires. However, grooves aligned perpendicular to the lane lines are not as effective in resisting bending stresses and stress concentrations created by the impact of tires. In this regard, the parallel grooves in the bottom surface of the pavement marker increases the potential for splitting the pavement marker along the parallel grooves in response to a load placed on the top surface of the pavement marker, such as a load attributable to the tire of a vehicle. A pavement marker that was reoriented so that the groves were aligned parallel to the lane lines or at an acute angle to the lane lines would maintain a desirably large surface area of contact and might reduce the chance of breakage due to a load applied to the top surface of the pavement marker. However, the retention attributable to the mechanical gripping of the adhesive would be reduced as compared to a pavement marker where the parallel continuous grooves were aligned perpendicular to the lane lines.
Other pavement markers have been molded to include an upper shell with a concave lower surface. The upper shell is inverted, and the concave interior of the pavement marker is filled with an epoxy potting material to a depth that generally conforms to the periphery of the bottom edge of the shell. Sand is embedded partly in the potting material so that the potting material exhibits a uniform roughness after curing. The cured potting material is retained securely in the shell and defines a hard substantially planar abrasive surface. The pavement marker with the cured potting material therein can be applied bottom-surface down onto the adhesive. The overall roughening achieved by the sand partly embedded in the surface of the potting material increases the surface area. The larger surface area improves the chemical bonding between the adhesive and the pavement marker in much the same way as the parallel grooves described above. Additionally, an enhanced mechanical gripping is provided between the adhesive and the leading edges of each grain of sand. Pavement markers of this type work well. However, the filling of the potting material into the inverted shell of the pavement marker and the need to cure the potting material adds significantly to the manufacturing time and cost. The finished pavement marker also is relatively heavy and hence contributes to shipping costs.
Some pavement markers include bottom surfaces with a circular array of grooves or with an array of discontinuous grooves that generally define a waffle pattern of recesses. The inventor herein has concluded that such designs trap air during the installation of the pavement marker, and hence minimize the area of engagement between the pavement marker and the adhesive. The reduction in the areas of engagement substantially reduce the ability of the adhesive to hold the pavement marker in place.
In view of the above, it is an object of the subject invention to provide a surface-mounted pavement marker with a bottom surface configured for secure retention on the surface of the pavement.